A voltage regulator which provides a regulated voltage may be a block in a circuit design. The voltage regulator may be part of an RF power amplifier. For instance for an RF power amplifier design the regulated voltage may be constant over supply voltage variation, have less temperature dependency, and have less dependency on load current. Further, the regulated voltage should be insensitive to process spread which may cause, e.g., a threshold voltage variation or a sheet resistance variation.
In GaAs (BiFET) technology it is difficult to design a voltage regulator with these properties, which is realized with minimum layout size, minimum current consumption, in on-state as well as in off-state, and low noise. Merged or stacked FET-HBT integration schemes, often called BiFET or BiHEMT and containing both HBT and FET or pHEMT devices on a single GaAs substrate, are reported in the following papers from the CS MANTECH Conference 2007: William Peatman, Mohsen Shokrani, Boris Gedzberg, Wojciech Krystek, and Michael Trippe: “InGaP-Plus™: Advanced GaAs BiFET Technology and Applications;” T. Henderson, J. Middleton, J. Mahoney, S. Varma, T. Rivers, C. Jordan, and B. Avrit: “High-Performance BiHEMT HBT/E-D pHEMT Integration;” Todd D. Basso and Richard B. Brown: “A Complementary GaAs Microprocessor for Space Applications;” Ravi Ramanathan, Mike Sun, Peter J. Zampardi, Andre G. Metzger, Vincent Ho, Cejun Wei, Peter Tran, Hongxiao Shao, Nick Cheng, Cristian Cismaru, Jiang Li, Shiaw Chang, Phil Thompson, Mark Kuhlman, Kenneth Weller: “Commercial Viability of a Merged HBT-FET (BiFET) Technology for GaAs Power Amplifiers;” C. K. Lin, T. C. Tsai, S. L. Yu, C. C. Chang, Y. T. Cho, J. C. Yuan, C. P. Ho, T. Y. Chou, J. H. Huang, M. C. Tu, and Y. C. Wang: “Monolithic Integration of E/D-mode pHEMT and InGaP HBT Technology on 150-mm GaAs Wafers.”
U.S. Patent Publication 2007/0159145 shows a voltage regulator in GaAs (BiFET) technology whose characteristics may vary with process spread, in particular the threshold voltage may vary.
Process spread variation, in particular threshold voltage spread, can be less of a problem if the process is within tight limits and monitored and controlled. But usually this is not enough, therefore process steering might be used which can affect other parameters and involve extra cost.
In the case of manufacturing a GaAs pHEMT the threshold voltage depends on the gate recess etching and on the epi starting material. The gate recess etching is extremely critical because it concerns nanometer scale accuracy, and process spread is therefore unavoidable. The epi starting material in GaAs technology often comes from an external supplier, so that the loop for epi process control is too long. Further, batch-to-batch variation can be large compared to wafer-to-wafer or on-wafer variation.
Yihong Dai, Donald T. Corner and David J. Corner: “A GaAs HBT bandgap voltage reference,” International Journal of Electronics, Vol. 92, No. 2, February 2005, pages 87-97, shows a complex circuit, among others a bandgap referenced voltage regulator circuit, which cannot meet all of the before-mentioned requirements, especially size, current consumption, noise performance, sensitivity for load-variations, etc., at the same time.
Another alternative is an externally supplied reference voltage. Although it might be possible to generate the regulator voltage in another part of the system, for power amplifiers the trend is to eliminate the external reference voltage.